1. Field of the Invention
The present invention relates to a hydraulic control circuit in a hydraulic excavator.
2. Description of the Related Art
A hydraulic excavator is used for various works and is required to have an operability suitable for each work. In a crane operation, an operability with a high gain control is required to operate precisely. On the other hand, in a parallel pulling operation (both boom raising and arm pulling operations are performed simultaneously, the motion of arm is relatively fast because the arm falls with its own weight and there is adopted a regeneration circuit), an operability with a high gain control is required to operate the arm fastly.
FIG. 8 is a hydraulic circuit diagram showing the prior art described in Japanese Patent Laid Open No. 189296/95. At the beginning of the parallel pulling operation in the hydraulic excavator disclosed therein, if an operating lever of a flow control valve 6 for boom is moved quickly, a pressure compensating valve 9 is throttled transitionally so as to greatly decrease the flow rate of pressure oil fed to an arm cylinder 4. As a result, even if the amount of of discharged from a pump is in a state of saturation and a boom cylinder 3 is an actuator for a high load pressure, a response delay of the pressure compensating valve 9 is remedied at the beginning of the parallel pulling work, whereby the fall of arm can be prevented.
FIG. 9 is a hydraulic circuit diagram showing the prior art described in Japanese Patent Laid Open No. 44234193. At the time of parallel pulling operation in the hydraulic excavator disclosed therein, if an arm dosing operation (arm pulling operation) is performed, a directional control valve 7 changes its flow passage, so that an oil pressure source 8' and a signal passage "c" are connected with each other, so that pressure oil passes through the signal passage "c" and causes a pilot check valve 6' to operate to its opening side. Consequently, the downstream side of an arm spool 3' bypasses a boom two-speed spool 2' and communicates with a tank T by means of the pilot check valve 6', so that the opening of the arm spool 3' is not interfered by the boom two-speed spool 2'. That is, in the arm dosing operation, a boom two-speed confluence is cut off to improve the parallel pulling performance.
The bleed-off of a main spool of a directional control valve for boom in a hydraulic excavator possesses a characteristic having a precise operability (i.e. low gain) and so is unsuitable for works for which high responsivity (i.e. high gain) is required. Therefore, various means have been proposed for improving the boom raising operability in works requiring a high responsivity (for example, the parallel pulling work).
In the hydraulic control system shown as an example of the prior art in FIG. 8, when the oil from a single hydraulic pump 2 is distributed to the flow control valve 6 for boom and a flow control valve 8 for arm, the distribution ratio can be changed. More particularly, in the parallel pulling operation, as the amount of operation of the flow control valve 6 for boom increases, the amount of rise of the boom increases in proportion thereto. Consequently, there arises a great difference between the amount of operation of the operating lever for boom and that of the operating lever for arm, and thus the boom raising operation is not easy.
In the prior art shown in FIG. 9, the boom two-speed confluence is cut off at the time of arm closing operation to prevent rising of the boom. Therefore, when the amount of operation of the arm operating lever is larger than the amount of operation of the boom operating lever, it is difficult to increase the boom raising speed.